The invention relates to time delay fuses.
Time delay fuses are used in connection with equipment having temporary current surges, such as motors and transformers. Time delay fuses often employ a fusible element and a spring-loaded heat mass. A deposit of solder retains the heat mass from movement by the spring. The dimensions of the fusible element are selected such that it melts quickly under short-circuit conditions (e.g., 30 times the rated current of the fuse). However, when lower overload conditions (e.g., 2 to 4 times rated current) persist for a predetermined amount of time, the solder instead melts, releasing the heat-mass to break the circuit.
Another approach used in time delay fuses utilizes the "M-effect," which is achieved by depositing a tin-bearing metal on the surface of a copper, silver, brass, or phosphor-bronze element such that the two metals alloy. The resulting alloy has a lower melting point than the element material alone. At low-overload conditions, the fusible element slowly generates heat. Eventually, the temperature rise is sufficient to melt the alloy region at the solder/tin deposit and thereby break the circuit. The time needed to generate the necessary heat results in a delay.